Ensuring the safety of antibiotics

Antibiotics, or antibacterials, are antimicrobial drugs used to treat or prevent bacterial infections. Antibiotics are heralded as the medicinal heroes of the 20th century. However, their effectiveness and accessibility has led to overuse and, in recent years, consequent antibiotic resistance.

Antibiotics are divided into classes based upon their method of production or bactericidal method of action. They may be produced biologically (fermentation), biologically and chemically (semi-synthetic), or by chemical synthesis alone.

Analysis of biological/natural product antibiotics

Antibiotics produced through fermentation processes are less predictable, less controllable and more complex than synthetic antibiotics. For this reason the variability in products derived from fermentation is often greater than products derived by chemical synthesis. The impurity profile of a fermentation product may also be more complex and less predictable than that of a synthetic product.

One of the most common antibiotic groups which is biologically synthesized are the aminoglycosides. An aminoglycoside is a molecule composed of a sugar group and an amino group.

Gentamicin C congener structures

Gentamicin

R1

R2

R3

C1a

H

H

H

C2

H

CH3

H

C2b

CH3

H

H

C2a

H

H

CH3

C1

CH3

H

H

Streptomycin was the first aminoglycoside antibiotic discovered and used in clinical therapy. These antibiotics are now widely used as clinical and veterinary medicines to treat bacterial infections because of their protein synthesis inhibition capability, leading to cell death. However, these antibiotics can have serious side effects and cause varying degrees of toxicity. It is important to develop sensitive and reliable analytical methods to characterize and quantify drug purity and detect minor degradants or impurities. Based on the nature of their production (fermentation), there are often mixtures of related components (congeners, isomers) and fractions which must be monitored and controlled.

As well as being structurally-related, many aminoglycoside antibiotics are actually synthesised from one another. For example, sisomicin is a broad spectrum aminoglycoside isolated from the fermentation broth of Micromonospora. Netilmicin is a semi-synthetic aminoglycoside antibiotic prepared from sisomicin. Both sisomicin and netilmicin are mainly used in the treatment of severe infections, particularly those resistant to gentamicin. Etimicin is semi-synthesized from gentamicin C1a, and so on.

Analysis of aminoglycosides and their related impurities is often achieved by ion-pairing reversed-phase (RP) high performance liquid chromatography (HPLC), based on their hydrophilic and positively charged nature. However, due to the lack of a suitable chromophore, aminoglycosides cannot be detected by ultraviolet (UV). Corona charged aerosol detectors (CAD), evaporative light scattering detectors (ELSD), mass spectrometers (MS), and electrochemical detectors are generally used to detect these compounds without prior derivatization.

Analysis of synthetic antibiotics

E.g., sulphonamides, quinolones, oxazolidinones

Chemically synthesised antibiotics often involve a number of intermediate compounds in their preparation, and these may remain as impurities in the final product. Chemical antibiotics are usually assayed by HPLC with UV detection, but where a chromophore is absent in either the active pharmaceutical ingredient (API) or impurities, IC with suppressed conductivity detection is considered the best alternative for selective determination.

Beta-lactam antibiotics are a class of broad-spectrum antibiotics, consisting of all antibiotic agents that contain a beta-lactam ring in their molecular structures. These antibiotics function through the inhibition of bacterial cell wall synthesis. Following extensive use, some bacterial populations have shown ability to develop resistance to beta-lactams and become more virulent.

Cephalosporins are a members of the β-Lactam antibiotics class, first discovered and isolated from the fungus Cephalosporum acremonium. The core structure of these antiobitics features a number of hydrogen bond donors and acceptor groups, as well as an acid function. Together these groups contribute towards the polarity of this class of compounds.

While beta-lactam antibiotics are similar to one another in many ways, they may differ in pharmacokinetics, antibacterial activity, and potential to cause serious allergic reactions. Some beta-lactam intermediate compounds and derivatives (from fermentation and/or synthesis) also possess similar sensitization and cross–reactivity properties. Beta-lactam intermediate compounds, such as β-lactam antibiotic API precursors, can undergo molecular changes or purification before use in manufacture. As a result of these changes, the intermediate compounds may develop antigenic characteristics that can produce allergic reactions. Drug manufacturers are required to take steps to control for the risk of cross-contamination and impurities for all beta-lactam products.

Separation of a mixture of cephalosporins on a Syncronis aQ LC column with 3 µm particle size. Good separation was achieved in a total run time of six minutes, with an elution window of 1.8 minutes for all four analytes. A controlled interaction mechanism and selectivity of the Syncronis aQ polar endcapping group is demonstrated.

Using the Chromeleon Chromatography Data System and simultaneous detection with UV and MS detectors provides complementary information for impurity profiling. The MS component channel with the extracted ion chromatograms of seven antibiotic standards was overlaid to the UV trace. The two traces could be perfectly aligned by taking into account the delay time between the two detectors. This simple comparison reveals the presence of an UV active impurity.

Often beta-lactamase inhibitors, such as clavulanate (clauvulanic acid) and sulbactam are co-formulated with beta-lactam antibiotics to increase their effectiveness through the counteraction of bacterial resistance. Impurities associated with the inhibitor as well as the antibiotic therefore need to be controlled and monitored.

Chromatographic comparison of clavulanate on the Dionex IonPac AS11 column A) without and B) with 4 µg/mL (0.8 %) 2-ethylhexanoic acid. This column has high capacity, low surface hydrophobicity and is able to separate a wide range of inorganic and organic anions in complex matrices.

Antibiotics in bioprocess of biopharmaceuticals

Antibiotics are heavily used in the production of biopharmaceuticals. Mammalian cell lines that express biotherapeutic proteins, such as antibodies, must be maintained over several weeks. They are fed with culture media supplemented with various vitamins, growth factors, and antibiotics to avoid contamination and growth failure. Testing of residual antibiotics in the product is required to ensure patient safety.

Analysis of a sample of cell lysate after tigecycline treatment (A) and the same sample spiked with 0.05 μg/mL tigecycline (B) using an on-line SPE-HPLC-UV method. This is a convenient method to determine trace amounts of tigecycline in tigecycline-treated cells which cannot be determined using a routine HPLC-UV detection method.